The immune response to HIV has largely been studied in the most accessible compartment - the peripheral blood, which contains only 2-3% of all lymphocytes. In contrast, at least half of the body's T cells reside in gut associated lymphoid tissue (GALT). Early in infection there is a rapid and profound depletion of CD4+ T cells in the intestinal mucosa with notable preservation of these cells in the peripheral circulation. Initiation of antiretroviral therapy typically results in the suppression of viral load and restoration of CD4+ T cells counts in the periphery, but the repopulation of CD4+ T cells in the gut is considerably delayed and incomplete. These findings suggest that the gut represents a unique sequestered viral niche. The functional characterization of HIV specific T cells in the gut, however, remains incomplete, particularly in "HIV controllers" - individuals who spontaneously control infection in the absence of antiretroviral therapy. In this application I propose to study the functional responses of intestinal CD8+ T cells from individuals with chronic progressive and controlled HIV infection using novel, state-of-the-art functional assays that will allow the multiparametric characterization of single cells. I hypothesize that the function of HIV specific CD8+ T cells is impaired in chronic progressive infection but preserved in controlled infection and that immune dysfunction in HIV infection is in part a consequence of HIV mediated disruption of the normal intestinal barrier, as well as alterations in intestinal dendritic cells. Specifically, I propose that E-cadherin, an adherens junction protein present on gut endothelial cells, mediates CTL impairment by binding to the inhibitory receptor KLRG1 on HIV specific CD8+ T cells. Additionally, I hypothesize that intestinal DCs have impaired HIV mediated Toll-like receptor (TLR) signaling, which consequently results in dysfunctional T cell homing to the small intestine in individuals with chronic progressive infection. In this application I intend to: 1. Determine the frequency and functional capacity of HIV specific gut mucosal CD8+ T cells in patients with progressive and controlled infection using novel assays of cytokine secretion and T cell killing, 2. Define the contribution of the gut microenvironment to T cell impairment by assessing the effects of E-cadherin binding to the inhibitory receptor KLRG1 on HIV specific CD8+ T cells, and 3. Investigate the role of HIV encoded TLR ligands in intestinal DC production of retinoic acid and the effect of retinoic acid on T cell gut homing receptor expression. RELEVANCE (See instructions): The majority of the body's immune cells reside in the gastrointestinal tract and this region is intensely active during HIV infection. The effects of HIV in the gut, however, remain incompletely studied. We are developing new, state-of-the-art techniques to better assess the immune system in the gut of patients with progressive HIV disease and in unique individuals who are able to control infection without medicines. These studies will provide insights that will assist in the development of more effective therapies and vaccines against HIV.